Echo suppressor using comb filters



March 23, 1965 C. C. CUTLER ECHO SUPPRESSOR USING COME FILTERS Arrow/Ey March 23, 1965 c. c. cUTLER 3,175,051

ECHO SUFPRESSOR USING COMB FILTERS Filed Dec. 16, 1960 A r TENUA r/oN (db) 2 Sheets-Sheet 2 FIG. 2A

fo f/ f2 f3 f4 fs f6 f7 fa N f FREQUENCY C/Rs ATTENUA r/oN Cla f, 12 1S f4 f5 f6 fr f FREQUENCY C/Rs ArrENUAr/ON fo nilbbbzbbwovz fb a fu 5 ff, f

f2 FREQUENCY C/Rs ATTENUA 7`/0N @2 nasa H 7:0 llmfb' f7 fa fs flo Nf/z f1.1 fu fis f /NVENTOR f/ f3 f5 FREQUENCY cs c C. CUTLER ATTORNEV 3,175,051 ECHO SUPPRESSQR USING @MB FLTERS Cassius C. Cutler, Gillette, NJ., assignor to Bell Telephone Laboratories, Incorporated, New York, NX., a corporation of New York Filed Dec. 16, 1969, Ser. No. 76,325 3 Claims. (Cl. 179-4762) This invention relates to an echo suppressor for use in two-way transmission systems involving both two-wirc and four-wire links and more particularly to echo Suppressors for use in such systems when the path delays are greater than those heretofore encountered. Asis well known in the art, it is the usual practice in telephone transmission Systems to interconnect two-Wire `local lines by way of four-wire circuits. Such a mixed system is particularly common where local terminals are "joined by way of radio facili-ties. The four-wire system provides two separate one-way paths for the transmission and the reception of signal energy between stations while t fthe two-wire local circuit provides a single two-way path. When energy is transmitted to a two-wire local line by one of the paths of the four-wire circuit, a part of the transmitted Signal returns over the other path of the fourwire system. This occurs even though the two facilities are interconnected by way of hybrid networks. This *characteristic of two-wire to four-wire communication systems gives rise to echoes since the signal energy is re- "Hturned to the Speaker'.

`It the returned echo signal has a delay ot less than 50 ,iilliseconda it is known that the echo will not present Iditliculties. However, certain transmission systems owl" proposed, such as that employing an earth satellite s "a repeater station, or a transmission system utilizing ongrlines for the transmission of the signal energy, can anse" delays of signalenergy up to 0.6 second. Failure d provide for attenuation ofthe echo will lead to disastrous results since the speaker using the system will always get an earful of the syllables of his last talk spurt. Echo Suppressors have accordingly been provided and have formed a partial solution to the problem under some circumstances. i

For example, so-called split echo Suppressors have been `used in prior transmission systems to reduce echoe suppressor being placed nearone talker,` th `pressor being placed near the othe talker egins to speak the `in means have been proor` use with these split echo Suppressors to enable ac `tall/rer to gain control of the circuit even `though the rother `person is speaking. However, this arrangement has the attendant disadvantage that any beneiit derived from the Suppressors will be lost as the echo will be allowed to return to the originating Speaker.

United States Patent O l'SlSl liatented Mar. 23, i965 ICC cannot distinguish the answer from the echo. Thus the break-in means can be actuated by the echo itself, again allowing the echo to come back to the speaker. The hangover time of these echo Suppressors could be increased to equal the round trip delay of the system; however, parts of the answer would be cutout. Therefore this solution is not feasible.

Another objection to the echo Suppressors now in use is that they require a Substantial amount of equipment, i.e., a detector to detect the signal, an amplifier to amplify the Signal and the means for disabling the return path.

Accordingly, it is an object of the present invention to` provide an echo Suppressor which will satisfactorily tenuate echoes in a combined two-wire and four-wir system having an inherent long delay time characteristic Another object of the present invention iS to reduc the complexity of echo Suppressors and to obtain improv performance with a minimum of equipment and attend circuitry for the operation thereof. In accordance with the present invention the basic ec o suppressor comprises two comb filters. These comb filters` have a response characteristic such that there are manf alternate stop (or attenuation) and pass bands throughout the frequency range for which the particular` filter is` designed. The attenuation bands of one comb filter ar chosen to be in alignment with the pass bands of th second comb filter for any given frequency in the ban l ot signal energy which is to be transmitted between th stations. One filter is placed in one of the transmiss' paths of the four-wire System; the other filter is place in the other transmission path of the four-wire system Thus any signal within the frequency range of `the filters will be attenuated during the round trl stations by either one or the other of the 'lte tore, any echo which might arise in th tw system and be returned `via the othe l wire system will be complet the` talker.

it Fill employed in the circuit of FIG. l; and

FG. 4 is a schematic circuit diagram of a modification of the invention wherein the comb lilters yare employed in combination with a conventional echo suppressor. Like characters of reference in the several figures of the drawing correspond to the same elements.

ln the following specification the speaker producing the March 23, 1965 w. E. LUDWIG 3,175,052

MULTIPLE-POSITION ROTARY STEPPING SWITCH HAVING AN CBLIQUELY CRIENTED CONTACT BRIDGE Filed July 20, 1961 ng! 1 P19/'0R Anf ,7g-2 

1. IN COMBINATION, A TWO-WAY COMMUNICATION SYSTEM COMPRISING FIRST AND SECOND TERMINAL STATIONS FOR THE TRANSMISSION OF SIGNAL ENERGY, A FIRST PATH FOR TRANSMISSION OF SIGNAL ENERGY FROM SAID FIRST TERMINAL STATION TO SAID SECOND STATION AND A SECOND PATH FOR TRANSMISSION OF SIGNAL ENERGY FROM SAID SECOND STATION TO SAID FIRST STATION, ECHO SUPPRESSING MEANS COMPRISING A VARIABLE ATTENUATOR CONNECTED IN SAID FIRST PATH TO INTRODUCE LOSS THEREIN IN PROPORTION TO THE LEVEL OF THE SIGNAL ENERGY IN SAID SECOND PATH AND A VARIABLE ATTENUATOR IN SAID SECOND PATH TO INTRODUCE LOSS THEREIN IN PROPORTION TO THE LEVEL OF THE SIGNAL ENERGY IN SAID FIRST PATH, AND ATTENUATING MEANS IN EACH OF SAID FIRST AND SECOND PATHS, SAID ATTENUATING MEANS HAVING EQUALLY SPACED ALTERNATING ATTENUATION AND PASS BANDS THROUGHOUT THE FREQUENCY SPECTRUM OF THE BAND OF SIGNAL ENERGY TO BE TRANSMITTED, THE ATTENUATION BANDS OF SAID ATTENUATING MEANS IN SAID FIRST PATH COINCIDING WITH THE PASS BANDS OF SAID ATTENUATING MEANS IN SAID SECOND PATH. 